Wave amplification and generation



Jan. 5, 1937. s, cARUTHERs 2,066,333

WAVE AMPLIFICATION AND GENERATION Bum 15 9 INVENTOR R. S. CARUTHERS JWWA T TORNEV Patented Jan. 5; 1937 WAVE AMPLIFICATION AND GENERATIONRobert S. Caruthers, .Monntain Lakes, N. .L, assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication December 14, 1934, Serial No. 157,422 8 Claims.(0!..179-1'11) The present invention relates to the transmission ofwaves in two circuits through the same circuit element such as anamplifier without mutual interference. Specifically, the inventionrehates to an amplifier which simultaneously amplifies waves in a givencircuit and generates oscillations of some desired frequencyindependently of its amplifying action on said waves.

An object of the invention is the simultaneous amplification of wavesand the production of oscillations by the same amplifier element.

If attempt be made to use an amplifierthat has an overloadcharacteristic, such as is exhibited by space discharge tubes, foramplifying waves and at the same time generating oscillations, itisfound that the circuit has a tendency to devote itself entirely toproducing oscillations rather than performing both of the intendedfunctions. It is a characteristic of an oscillating system to reach asteady state condition in which the loss in the system is just balancedby the gain. Initially the oscillations start and build upin amplitudeuntil this steady state condition is reached, Usually the limit is fixedby the overload characteristic of the tube or other amplifying element.As the amplitude of the oscillations reaches the overload point the tubelosses increase at a steeper rate, or looked at from the standpoint ofgain, the gain falls off, with the result that further increase inamplitude is checked and the circuit settles down to an oscillatingcondition at an amplitude,limited by this overload characteristic. Inthe usual oscillation generator circuit there is nothing to prevent theoscillations from increasing up to the limit set by the overload pointof the tube. If, then, otherwaves are impressedon the same circuit theycannot be amplified by the tube since there is no margin of loadcarrying capacity left for the tube over and above that used foroscillation production.

In accordance with the present invention, a circuit is provided with anoverload or limiting means which serves to limit the amplitude of theoscillations being generated at a value well below the overload orload-limiting point of the amplifier element, thus leaving aload-carrying margin which can be used for amplifying other waves.

Such a limiting means may take any one of a variety of forms and may beof simple construction. A piece of substance known to the trade asThyrite or a copper oxide rectifier or, in general, any means whichexhibits an overload or non-linear relation between current and voltagemay be used.

The nature of the invention and its various features and objects willappear more fully in the detailed description to follow, together withbattery l3, choke coil the drawings forming a part of thisspecification.

In the drawings:

Figs. 1, 2, 3, 4, 5 and 8 are schematic circuit diagrams of variousforms of circuit for both amplifying waves and generating oscillationsin accordance with the invention; 7

Figs. 1A and 1B show detail views of different types of non-linearresistances; and- Figs. 6 and '7 show the application of the waveamplifying and generating circuit to carrier systems according to theinvention.

Referring first to Fig. -1 the pentode vacuum tube H1 is arranged as acombined amplifier and oscillator. For its amplifying function it has aninput transformer II and an output transformer I! for connectingrespectively to any suitable input circuit and output circuit. The wavesthat are being amplified may be speech waves or waves of any othersuitable type orfrequency range and it is assumed that the transformersII and I2 and their associated connecting circuits are of suitabledesign to accommodate the type and frequency of the waves beingamplified. Space current for the tube is supplied from a plate batteryI3 which also supplies positive potential for the screen. Negative gridbias potential is obtained as the drop across resistance It, the gridend of which is connected to the minus pole of the plate battery l3,this connection being indicated in the drawings by grounds.

The grid circuit for the wave being amplified is traced from the gridthrough the secondary winding ofinput transformer ll, transformerwinding I5, bias resistor l4 to the cathode. The output circuit for thespeech or other waves being amplified may be traced from the anodethrough the primary winding of output transformer l2, winding l1,condenser [6 (which may be as large as is required to accommodate thewaves being amplified), to the cathode. The path for they traced fromground through l9, winding ll, primary of output transformer I! to theanode, thence to space current may be the cathode, through biasresistance It to ground.

The tube 10 generates oscillations by virtue of the feed-back from theplate tuned circuitcomprising the condenser I8 and inductance l1 and thewinding l5 which is in the grid circuit. Condenser I8 may be variable asshown to control the frequency of the waves being generated. The-- blockindicated 5 in the drawings is in shunt to the winding [5 and thereforeeffectively in shunt to the tuned circuit l1, Hi. This element fS of thecircuit will be described more fully in connection with Figs. 1A and 13.It may comprise any suitable element having a non-linear current voltagecharacteristic or an overload characteristic such as to place an upperlimit on the'amplitude of the oscillations corresponding to a pointbelow the overload point of the tube II.

In considering the operation of the circuit of Fig. l, as an oscillationgenerator, the oscillations may be thought of as starting from a verylow value and building up towards the steady state condition. For low.amplitude oscillations the element S has very high impedance so thatsubstantially the full voltage of the oscillations generated in the coilI5 is applied to the grid. After 7 the oscillations have reached acertain value, however, the effective resistance of the element S fallsto a low value and any tendency towards a further increase in theoscillations is counteracted by the tendency ofthe element S to fall toa still lower resistance; thus limiting the oscillation voltage that isapplied to the grid. The oscillations therefore are limited in theirmaximum amplitude by the operation of the element S. This element isproportioned to limit the oscillations to a value less thanthey wouldhave if the element S were absent and if the tube ill performed thelimiting function as in the case of the ordinary oscillationgenerator.The limit set to these oscillationsmay be sufficiently low to permit thetube III to operate eflectively as an amplifier for the waves impressedon the tube through the input transformer. ll'.

The details of one type of circuit which may comprise the element S areshown in Fig. 1A. In this case two copper oxide rectifiers poled inrespectively opposite directions are connected in parallel between theterminals of the element S.

In some instances'it may be found sufilcient to mitted through therectifier in question develops a voltage across resistance 22 which isstored in shunt capacity 2 I. This voltage serves to polarize thecorresponding rectifier element. The polarizing or, bias potentialapplied tothe rectifier aids in determining the overload point of therectifier. It also increases the sharpness of theoverload characteristicof the rectifiers as the oscillation builds up. This is very importantfrom the standpoint of stability.

A second type of element S is shown in Fig. 1B and comprises an element23 of a material known in the trade as Thyrite, comprising a mass ofsilicon carbide crystals and a suitable binder as specifically describedand claimed in U. S. patent to McEachron 1,822,742 granted September 8,1931. This material has a non-linear resistance characteristic.

The invention is not limited to the use of the two types of non-linearresistances shown in Figs. 1A andlB since any suitable type ,ofnon-linear resistance may be used, including other than solid materials,for example a space discharge device.

The circuit of Fig. 1 represents one type of series feed-back in whichthe feed-back circuit is in series with both the output and the inputcircuits of the tube In. This type of circuit has been referred to inthe. art as series-series" feedback. Other types of feed-back circuitsare possible-in which the feed-back connection on the output side may beeither series or parallel and on the input side may be either series orparallel, making possible the types of feed-back (in addition to theseries-series type) which may be identified as the shunt-series,theseries-shunt and the shunt-shunt types. It will be noted that in thecircuit of Fig. '1 the oscillations that are generated by the tube IIare applied to the output circuit through the transformer l2 but theyare substantially kept out of the input circuit because of the seriesrelationship between'the feed-back connection on the input side and thesecondary of. the input coil ll.

Fig. 2 representsa type of shunt-shunt feedback, that is, one in whichthe feed-back circuit is in shunt to the amplifier circuit on both itsoutput and its input sides. In this figure various elements areidentified with corresponding 'elements of Fig. 1' by the use ofsimilarreference characters. The essential diflerence is that the feed-backcircuit in this figure .is connected in parallel to the output and tothe'input' circuit,

specifically outside of the coils l'l ens mthct'a.

on the side remote from the tube-i0. This feed-= back circuit could,however, be connected these coils with generally-.similar'aresults. The?feed-back circuit includes resistance elements 24" and'25 offering ahigh impedanceto the mission of current directly between the input andoutput circuits. It also includes the parallelcombination of inductance28 and condenser 21 which together form the frequency determiningelement of the oscillating system, and the limiting element S, which isalso in shuntsof the feed-back path.

. By virtue of this connect-ion, the waveswhich are efiectivelytransferred from the output to the input circuit are waveshaving afrequency corresponding to the anti-resonantfrequency of the circuit 26,21 and having an amplitude below a certain maximum which is determinedby'the point at which the shunt element S begins tointroducesuificiently low conductivity. The circuit, therefore, generatesoscillations of amplitude below the overload point of the element 8which, by design, may be made sufilciently lower than the overload pointof the tube III to permit this tube also to amplify other waves fromtheinput into the output circuit. If it is desired that the oscillationsbe transmitted in both directions from the tube I0, that is, both intothe inputand into the output circuit, a shunt type of connection, oneexample of which is. shown in Fig. 2, Y

may be used. e Fig. 3 shows a modified type of shunt feedback circuitincluding a bridge comprising three resistance arms 2|, 32, 33 with theelement 8 as the fourtharm of the bridge. The frequencydeterminingcircuit is the resonantparallel combination, inductance 20and capacity 21, this being effectively isolated from the output by asuitable resistance pad 30: which prevents the resonant circuit fromappearing as effectively shunted across the output where .it mightinfluence the transmission of waves in an undesired manner. By properlyproportioning the resistances ll, 32 and 33 and the element S, thebridge may be unbalanced for low amplitudes of. the current beinggenerated, thus effectively feeding this current back to the input; Asthe amplitude of current in the feed-back circuit increases, however,the bridge approaches a condition of balance by virtue of the change inresistance of the element 8 so that further increases in amplitude tend.to feed back less'and less to the input. The limiting point .is reachedas the result of the two-fold action of the bridge and of ,thenon-linear re- 7 the input bridge the circuit could be made togensistance of the element 8. By making some of the resistances 3|, 32,33 variable or by choosing proper values for these resistances the pointof balance of the bridge may be controlled and thus the limit ofamplitude of currents generated may be controlled. As in the otherfigures the tube Ill may serve forthe amplification of waves of anydesired type independently of the currents genor balancing resistance36. In the case of the output bridge, resistance 4| shunting the outputof coil l2 forms one arm of the bridge, resistance 44 forms the oppositearm while the other two 1 arms are comprised of resistances 42 and 43.For a perfect balance between the sides a. and c of erate oscillationswithout substantially any of these oscillations getting into the inputcircuit. A balance between sides a and c of the output bridge preventscurrents in the output circuit beyond the bridge from-getting back intothe feedback circuit. Currents fiow into the output circuit, however,from b to a. By a choice of ratio arms the loss between the sides a andb of each bridge may be made low while the loss between sides b and 0may be made high. Thus the amount of current flowing in the feed-backpath may be made small in comparison with that supplied to the outputcircuit. Speech waves or other signals transmitted through the circuitare effectively transmitted to the input of the amplifier Ill and fromthe output side of this amplifier into the output circuit.

In each of the circuits that have been described, if the ratio ofreactance to resistance of the tuned circuit 26, 21 is made high thiscircuit becomes a low impedance shunt to currents of all frequenciesexcept the resonant frequency. Currents" other than the resonantfrequency are,

therefore, efiectively prevented from being fed back.

In Fig. 5 the tubes 53 and 54, each of which may be similar to tube Inof the other figures or of any other suitable type, are connected inpushpull relation as regards the input circuit connected to input .coil5! and theoutput circuit connected to output coil 52. These tubesamplify waves of any suitable type impressed on the input circuit,acting as a push-pull amplifier for this purpose. Tubes 53 and 54generate oscillations in a parallel circuit by virtue of a feed-backcircuit connecting the common branch of the output'circuit with thecommon branch of they input circuit. This feed-back circuit is tracedthrough large condenser 55, resistance 56 and inductive winding 51, thelatter of which is coupled to the inductance 58 of the tuned circuitcomprising inductance 58 and condenser 59. The limiting element S isconnected between the common branch of the input circuit and anadjustable point along the resistance 56. The oscillations may be takenoff through output winding 60 and applied to any suitable load circuit.The load limiting element S is virtually in shunt to the tuned circuit58, 59 since it is connected across the primary winding 51 in somewhatthe same manner as in Fig. 1. The amount of oscillating current voltageimpressed across the terminals, of the device 8 is controlled by theposition of the slider on the resistance 56 so that for a given type ofelement S the amplitude of the oscillations generated before thelimiting action of the element 8 sets in may be controlled by adjustmentof the slider on the resistance 56.

Fig. 6 indicates-a terminal of a carrier telephone system comprising aneastward multiplex line 12 and a westward multiplex line 13. One branchfor each line is shown in Fig. 6 and may be similar to other branches.For the transmitting line 1! band filter l6 leads to a-modulatingcircuit connected on the other side to low I frequency line 10 which maylead to an exchange where it is extended as a voice frequency line oneither a four-wire or two-wire basis.

' The westward or receiving line is shown with band filter 'l'l leadingthrough resistance bridge network 8i and receiving amplifier ID todemodulator 19, the output of which is connected through low-pass filterI5 to voice line II which may lead to the same point as voice line Ill.These two voice lines 10 and II may be considered as eventuallyconnected to a subscribers line for two-way talking.

The modulator l8 and the demodulator 19 are each shown as of the bridgetype employing nonlinear resistances which may be copper oxiderectifiers, for example. The speech is applied across one diagonal ofthe bridge while the carrier used for modulating or demodulatingpurposes and, in the case of demodulator 19, the sideband, are appliedacross the opposite diagonal.

The receiving amplifier I0 is provided with a feed-back circuitcomprising frequency-determining combination 26, 21 and element S in acircuit similar to that of Fig. 4 except that the bridge of Fig. 4 isomitted in this figure on the output side of the amplifier. Theamplifier Ill, therefore, serves as an oscillation generator producingwaves of carrier frequency which are applied to the modulator 18 and thedemodulator 19. The bridge comprising ratio arms and 8! and as its otherarms modulator l8 and input of amplifier i0 is balanced so that theoscillations generated by the tube in are not applied to band filter Ti.The bridge is. preferably adjusted so that the loss from the feed-backpath into modulator I8 is low whereas the loss into the input ofamplifier I0 is high. It is assumed that the same frequency carrier waveis used for a given channel on each line 12 or 13.

The operation of the circuit of Fig. 6 is as follows: Speech wavescoming from the speech line to which line 10 is connected pass throughlowpass filter 14 and modulate in the modulator 18 the carrier wavesupplied from the amplifieroscillator circuit I 0, 26, 21. Modulator I8is balanced so that the unmodulated carrier'component is nottransmitted. To aid in securing this balance a potentiometer is includedbetween two of the copper oxide elements as shown,-equipped with aslider to which one of the carrier input terminals is connected. Onesideband of the resulting modulated wave is transmitted from resistancenetwork 83 through band-pass filter 16 into the outgoing line 12.Resistance network 83 is a pad preventing transmission irregularitiesdue to interaction of copper oxide and band filter reactances. Insimilar fashion waves from other lines similar to line 10 in otherchannels are used to modulate carrier waves of other frequencies' andthe resulting sideband frequencies 75 are transmitted through other bandfilters to the same line I2.

Bya terminal circuit which may be identically lator 19. Some of thecarrier wave generated in the circuit comprising'amplifier I9 isimpressed together with the sideband components on the de-v modulator19. The demodulated voice frequency components are then transmittedthrough the low-pass filter l5 and impressed upon the voice frequencyline ll. The bridge including resistances 80 and 8| may be soproportioned that the loss from the output of band filter 1'! to theinput of amplifier I is low whereas the 10$ from the band filter 11 intothe modulator I9 is high..

A feature of considerable interest and importance in connection with acircuit of the type shown in Fig 6, where the combinationamplifieroscillator l0 feeds intoa non-linear resistance circuit such as19, is that the element S may be omitted because of thenon-linearresistance characteristic of circuit 19; and the amplifier Ill may bemade to perform the two-fold function of generating oscillations andamplifying waves. For this purpose the non-linear circuit 19 furnishesthe overload characteristic for determining the maximum amplitude of theoscillations generated in the circuit in the same manner as is describedhereinbefore in connection with the element S. This represents asimplification.

Fig. 7 discloses a circuit generally similar to that of Fig. 6 butcapable of'greater accuracy in the frequency of the generated carrierwave. The type of oscillator circuit disclosed is essentially that ofFig. 2, a shunt type, but it includes a crystal 91 for accuratelydetermining the frequency of the waves generated. It also makes use ofthe fact that the impedances of coils H and I! are fairly pure capacityreactances at carrier frequencies on the sides facing the tube.Condenser i8 and coil 9| are arranged so that small shunting action isinserted across the high winding of coil I! at voice frequencies. Alsothe junction between condenser I9 and coil 9| is efiectively connectedto the plate at carrier frequency and to cathode at voice frequency.This allows feed-back at carrier but none at voice frequency. Crystal 91lies in the feed-back connection from the junction of coil 9| andcondenser to the grid. This makes for greater constancy of frequency ofoscillations generated, as for example, with temperature changes. Theoutput of tube III for the generated oscillations is from winding 9i toinductively coupled coil 92 which is connected to modulator 19 anddemodulator I9.

The space current circuit for the tube l0 may be traced from groundthrough battery l9, choke coil I9, primary .output coil II to the anodeof tube l0, thence to the cathode, through oneside of circuit I00,resistance 94, opposite side of circuit I99, resistance ll, for gridbias, back to ground. It is thus seen that both resistor I4 and variableresistance 94 are included between the cathode and .ground or minus "3.The path that is traversed by speech waves is from anode fed backreversely on the grid.

through primary winding;of--outputcoii l2,-condenser I02 and resistance94 to the cathode, so

that resistance 94 represents a coupling from.

plate to grid circuit for speech waves, this cou-' pling being of suchsign as to reduce the degree of amplification for the speech waves. Theresistance 94 being variable offers a control for the gain of theamplifier tube Ill since variations in this resistance control theamount of voltage of. voice frequency (as well as direct current) thatis The leads ill may be extended to a convenient point for mounting thecontrol 94 alongwith similar controls for other receiving channels.

It will be noted that the copper oxide rectifiers in modulator 18 are sopoled with respect to those in demodulator 19 that carrier waves appliedto both modulator and demodulator from the coil, 92 fiow alternatelythrough 19 and 19 in opposite.

half waves of the carrier. This form of connectio n of the modulator anddemodulator to the carrier supply circuit afiords an impedance which isfavorable to the suppression of second order I harmonics from thecarrier jsupply circuit; 1h J sometypes of modulators such as those'emplo'y.

ing copper oxide rectifiers the second harmonic S 1 frequently has thelargest amplitude of any ha'r-' monic. For efiiciency reasons it isadvantageous j to have this relatively strong harmonic currentdissipated by circulating through the modulator and demodulator circuitas is done in thetype of circuit of Fig. 7.

The operation of Fig. 7 is generally similar to that described in Fig.6. Speech waves in line It are transmitted through the modulatingapparatus and eventually into eastward carrier line 12 in the samemanner as described in Fig. 6.

Modulated carrier waves received over line 13 from the opposite stationpass through band filter I1 and are demodulated at I9. The inductances99 ofler high impedance to the sideband current in shunt of themodulator 19 but permit the possage of speech waves with low loss. Theresulting speech waves are impressed on the amplifier l0 through inputcoil H and from the output of the amplifier l9 they pass through outputcoil l2, low-pass filter 15 into line H. By varying resistance 94 thegain of the amplifier It for the speech waves may be varied. The tube l0continually produces oscillations of the carrier frequency as determinedby the crystal 91 and the carrier frequency waves are supplied to boththe modulator l8 and demodulator '19. .The nonlinear impedance whichlimits the maximum amplitude of the generated carrier oscillations isthat of modulator I8 (Jr-demodulator 19 which are effectively connectedacross 9|, by virtue of the coupling of coil 92 to this circuit. Themaximum amplitude of the oscillations is limited to a point suificientlybelow the overload point of amplifier It to permit the emcientamplification of the detected speech waves.

In a carrier system for several channels in each direction employing vaterminal circuit of the type shown in either Fig. 6 or Fig. 7 eachtrans-' mitting channel at a station is paired with a correspondingreceiving channel, the same carrier frequency wave is used for bothchannels and is produced in a common oscillating circuit as disclosed.Oscillating circuits of identically the same carrier frequency are, ofcourse. employed. at the opposite terminal for each pair of chan' nels.

Fig. 8 shows a type of circuit in which the nonlinear resistance used tolimit the amplitude of the oscillations being generated comprises spacedischarge paths included preferably in the same tube with the amplifyingelements. The type of tube I III disclosed for this purpose is known asa duplex diode pentode tube, and is preferably provided with an internalshield I II effectively separating the portion of the bulb including thediode plates I I2 and H3 from the portion including the usual pentodeelements such as control grid, space charge grid, screen grid and anode.The same cathode I I4 may serve for both sets of electrodes and is shownextending through the shield H2.

The operation of the circuit is generally similar to that of the earlierfigures. Input coil II and output coil I2 enable the tube to beconnected between input and output circuits for amplification of wavesas in the other figures. Condenser I02 is large enough to pass readilythe wave being amplified. Tuned circuit I8, I! determines the frequencyof the oscillations generated, the feedback path comprising secondarywinding I5 and resistances 24 and 25 somewhat as in Fig. 2. For lowamplitude waves in the feed-back circuit the space paths between anodeH2 and cathode H4 andbetween anode I I3 and cathode I I4 offer highshunt impedance and the oscillations build up to a point where thesespace paths introduce a limiting effect. In other words one or the othersuch space path is a non-linear resistance in shunt to the half of thecoil I5 which is at any instant driving the anode I I2 or I I3 positivewith respect to cathode II4, the shunt path being completed through thelead 5 and one of the resistances 24. Such non-linear shunt resistancelimits the value of the oscillation current that is allowed to build upin the feed-back path. By proper design the space paths II 2-I I4 andIl3-I I4 can be made to reach a low resistance point well below theoverload-point of the tube H0 for the waves which are being amplified byit. The shield III prevents undesired lnfiuence of the discharge spacesII2-I I4 and II3-I I4 upon the elements of the tube used foramplification.

I The circuit of Fig. 8 may be used alternatively to the circuits thatare shown provided with an overload device S, in the carrier systems ofFigs. 6 and '7.

The circuits that have been shown and described are to be taken asillustrative rather than as limiting, since the invention is capable ofembodiment in many forms including forms other than those specificallyshown. The scope of the invention is defined in the claims.

What is claimed is:

1. In combination a space discharge tube having an input and an outputcircuit, a feedback coupling for causing said tube to act as a generatorof sustained oscillations, means toutilize the generated oscillations, aresistor of non-linear characteristic connected to said tube to limitthe amplitude of the generated oscillations to a value below thatcorresponding to the limit of the loadcarrying capacity of the tube,whereby a portion of the load-carrying capacity is left unused in theproduction of the oscillations, means to impress waves. independently ofthe oscillations being generated, upon the input circuit to be amplifiedby said tube, and means in the output cirlations but outside the pathtraversed by said waves.

3. In combination, a circuit traversed by waves, an amplifiereffectively in said circuit for amplifying said waves, a feed-back pathfor said amplifier for causing the amplifier to generate oscillationsand a current limiting means operative to limit the amplitude of theoscillations, said means being effectively outside of the path traversedby said waves.

4. In combination, an amplifier having an input circuit and an ouputcircuit, a source of waves to be amplified connected to said inputcircuit and a utilization circuit for the amplified waves connected tosaid output circuit, a feed-back path forming with said amplifier andits input and output circuits a system for generating oscillationsindependently of its function of amplifying waves from said source,means to utilize the generated oscillations, and a non-linear resistanceeifectively in the oscillation generating system for limiting theamplitude of the generated oscillations.

5. In combination, a main circuit transmitting waves, an amplifierhaving an input circuit and an output circuit effectively connected tosaid main circuit to amplify the waves transmitted by it, a feed-backcircuit from the output to the input of said amplifier for causing theamplifier to generateoscillations, means to utilize the generatedoscillations, means limiting the amplitude of the generated oscillationsto a value below that corresponding to the limit of the load-carryingcapacity of said amplifier, said feed-back path being connected to theinput circuit of said amplifler in parallel relation-with the maincircuit. 6. In combination, a main circuit transmitting waves, anamplifier having an input circuit and an output circuiteifectivelyconnected to said main circuit to amplify the wavestransmitted by it, a feed-back circuit from the output to the input ofsaid amplifier for causing the amplifier to generate oscillations, meanslimiting the amplitude of the generated oscillations to a value belowthat corresponding to the limit of the loadcarrying capacity of saidamplifier. said feed-back path being connected to the input circuit ofsaid amplifier in series relation to the main circuit.

'7. In combination, a main circuit transmitting waves. an amplifierhaving an input ,circuit and an output circuit efiectively connected tosaid main circuit to amplify the waves transmitted by it, a. feed-backcircuit from the output to the input of said amplifier for causing theamplifier to generate oscillations, means limiting the amplitude of thegenerated oscillations to a value below that corresponding to the limitof the loadcarrying capacity of said amplifier, said feed-back pathbeing connected to the output circuit of said amplifier in parallelrelation to said main circuit.

8. In combination, a main circuit transmitting waves, an amplifierhaving an input circuit and an output circuit eifectively connected tosaid main circuit to amplify the waves transmitted by it, a feed-backcircuit from the output to the input of said amplifier for causing theamplifier to generate oscillations, means to utilize the generatedoscillations, means limiting the amplitude of the generated oscillationsto a value below that corresponding to the limit of the load-carryingcapacity of said amplifier. said feed-back path being connected to theoutput circuit of said am.. plificr in series relation to said maincircuit.

ROBERT B. CARUTHERS.

